Alkanolamine sweetening units are used for the removal of H.sub.2 S and CO.sub.2 from natural gases, enhanced oil recovery gases, refinery hydrodesulfurizer recycle gases, FCCU and Coker gas plant tail gases, LPG streams, and Claus sulfur recovery tail gases. The alkanolamines commonly used are ethanolamine, diethanolamine, methyl diethanolamine, diisopropanol amine, and triethanolamine. These compounds are weak bases in water solution. When solutions of alkanolamines are contacted in packed, sieve plate, bubble cap, or valve tray columns with streams containing H.sub.2 S and CO.sub.2, the H.sub.2 S and CO.sub.2 dissolve into the alkanolamine solution The following chemical reactions then take place: EQU H.sub.2 S+AAmine=AAmineH.sup.+ HS.sup.- EQU H.sub.2 O+CO.sub.2 +AAmine=AAmineH.sup.+ +HCO.sub.3.sup.- EQU General Eqn. Acid Gases+Alkanolamine=Alkanolamine Salts of Acid Gases
The solution of water, unreacted alkanolamine, and alkanolamine salts are subjected to steam stripping to decompose the alkanolamine salts and remove H.sub.2 S and CO.sub.2 from the alkanolamine. The H.sub.2 S and CO.sub.2 removed from the alkanolamine can then be processed by Claus sulfur recovery, incineration, fertilizer manufacture, or other means.
H.sub.2 S and CO.sub.2 are not the only gases in the above referred to streams which form weak acids when dissolved in water. Other such acid gases, as they are commonly called, that may appear in gas streams treated with alkanolamine include SO.sub.2, COS, or HCN. These gases also undergo the same reactions as H.sub.2 S and CO.sub.2 to form alkanolamine salts. These salts, though, cannot be removed by steam stripping as H.sub.2 S and CO.sub.2 salts are. Thus, they remain and accumulate in the system.
Another problem is presented if oxygen gets into the alkanolamine system. Oxidation of acid gas conjugate base anions leads to the formation of other alkanolamine salts, most commonly salts of thiosulfate (S.sub.2 O.sub.3.sup.-2), sulfate (SO.sub.4.sup.-2) and thiocyanate (SCN.sup.-). Other inorganic acid anions such as chloride (Cl.sup.-) may also be present. In addition to the inorganic acid anions, the alkanolamine solution may also be contaminated with organic anions such as anions of formic and acetic acid and the like. The alkanolamine salts of these inorganic and organic anions also cannot be regenerated by steam stripping.
Alkanolamine salts which cannot be heat regenerated, called heat-stable salts, reduce the effectiveness of alkanolamine treating. The alkanolamine is protonated and cannot react with either H.sub.2 S or CO.sub.2 which dissolve into the solution. Also, accumulated alkanolamine salts are known to cause corrosion in carbon steel equipment which is normally used in amine systems. The salts are also known to cause foaming problems which further decrease treating capacity.
The alkanolamine treating process is conducted in metal vessels and lines which are subject to corrosion by the various chemicals passing therethrough. As a result of the corrosion small amounts of iron sulfide are released in the system.
Typically, iron sulfide is removed from the alkanolamine solution by filtration. The filter media has openings which allow the liquid to move through the openings but restrict particles larger than the openings from passing through the openings. Several filter media and filter operations are available. Some filters are single layers of media which stop all particles larger than the opening. They continue to filter by building a cake of the material on the media; the fluid is pushed through the cake by pressure differential, and the particles stick on the top layer of cake. Other filters use fibers as a filter media. The fibers are bound in a three-dimensional structure. As the alkanolamine containing iron sulfide enters the fiber structure, the particles become trapped because the openings between the fibers in the structure are smaller than the particles. The particles remain trapped in the structure increasing the pressure differential through the media.
Both types of filters must be cleaned to remain effective. Both filters are usually cleaned after reaching some predetermined pressure differential caused by buildup of particles in the structure or depth of cake. Single layer (cake filtration) filters can be cleaned by scraping the cake off or backflushing the media to push the cake off. Structured fiber elements are simply replaced with clean ones. The scraped and backflushed filters are reusable.
It would be desirable to provide a process for the removal of iron sulfide particles from alkanolamine solution which is not restricted by pressure drop and in which mechanical cleaning of filter elements is not required.